Pneumocystis carinii (Pc) remains a major cause of morbidity and occasional mortality in HIV infected patients. Bronchoscopy with bronchoalveolar lavage (BAL) allows for studying the lung in symptomatic patients. With lavage, the diagnosis of Pc is usually made. In addition, lavage provides a source of Pc specimen to be studied. We plan to continue our policy of collecting and handling BAL specimens in a standardized fashion at our institution. This standard analysis has provided useful clinical information, and has been used to provide samples for research use. The proposed research will examine BAL samples in three areas: organism analysis (including quantitation, viability, and molecular karyotyping), immunologic identification (evaluation of monoclonal and polyclonal antibodies against various antigens of rat and human Pc), and immune response (as assessed by the inflammatory nature of the lavage cells as well as systemic antibodies). These techniques, along with the clinical information obtained on these patients will allow to study these hypotheses: (1) BAL remains useful in diagnosing Pc pneumonia in HIV infected patients; (2) Follow-up BAL is useful in assessing response to therapy; (3) Analysis of local and systemic immune response to Pc pneumonia provides diagnostic and prognostic information; (4) Recurrent Pc infection represents reinfection rather than reactivation of latent organisms; (5) The characterization of the Pc isolates will predict virulence and antimicrobial sensitivity; (6) Characteristics of Pc isolates vary with geographical and sociologic conditions.